Peelable metal protectants

ABSTRACT

A LOW SOLVENT, HIGH SOLIDS CONTAINING MASKANT OR METAL PROTECTANT FILM FORMING LIQUID COMPOSITION INCLUDING A LOW MOLECULAR WEIGHT LIQUID DIENE PREPOLYMER HAVING END AND SIDE CHAIN FUNCTIONALITY SUCH AS A 10002000 EQUIVALENT WEIGHT HYDROXYL, THIO, EPOXY, AMINE, CARBOXYL, CHLORO OR ISOCYANATE TERMINATED AND SIDE CHAIN MODIFIED DIENE POLYMER HAVING A FUNCTIONALITY OF AT LEAST 2 AND A CROSS-LINKING AND CURING AGENT SUCH AS AN ALIPHATIC, CYCLOALIPHATIC OR AROMATIC POLYISOCYANATE, POLYAMINE, POLYOL, OR POLYBASIC ACID, ACID CHLORIDE OR ANHYDRIDE. SAID COMPOSITION BEING CURABLE TO FORM A PEELABLE POLYMERIC FILM HAVING A MOLECULAR WEIGHT GREATER THAN 20,000. THE MASKANT FILM MAY CONTAIN ALUMINUM, MAGNESIUM OR CALCIUM CARBONATE, OXIDE OR SILICATE FILLERS SUCH THAT THE FILM IS TRANSLUCENT AS APPLIED, BUT BECOMES VISIBLE IN REGIONS THAT ARE MECHANICALLY STRESSED.

United States Patent Ser. No. 722,156

Int. Cl. C23b 3/04 US. Cl. 260-415 R Claims ABSTRACT OF THE DISCLOSURE A low solvent, high solids containing maskant or metal protectant film forming liquid composition including a low molecular weight liquid diene prepolymer having end and side chain functionality such as a 1000- 2000 equivalent Weight hydroxyl, thiol, epoxy, amine, carboxyl, chloro or isocyanate terminated and side chain modified diene polymer having a functionality of at least 2 and a cross-inking and curing agent such as an aliphatic, cycloaliphatic or aromatic polyisocyanate, polyamine, polyol, or polybasic acid, acid chloride or anhydride. Said composition being curable to form a peelable polymeric film having a molecular weight greater than 20,000. The maskant film may contain aluminum, magnesium or calcium carbonate, oxide or silicate fillers such that the film is translucent as applied, but becomes visible in regions that are mechanically stressed.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of my copending application Ser. No. 642,198, entitled Method and maskant Composition for Chemical Milling or Plating, filed on May 29, 1967, now US. Pat. 3,544,400.

BACKGROUND OF THE INVENTION 1) Field of the invention The present invention relates to maskant forming and metal protecting liquid compositions and to methods of preparing and utilizing such compositions in chemical milling, plating and fabricating metal articles.

(2) Description of the prior art A sophisticated art of precision, selective etching or plating of metal workpieces known as chemical milling has developed in which portions of a metal object are removed or additional metal is added to obtain an article having a desired structural or ornamental configuration. The metal pieces are coated with several layers of a liquid maskant forming material'which, when cured, provides a resilient, peelable film resistant to the etchant or plating bath employed in the process. The maskant is then scribed and cut through with a sharp knife in the selected area to be etched or plated and this portion is removed from the maskant by peeling.

The currently used chemical milling maskants are prepared from relatively low solids solutions or dispersions generally containing less than about 45% of high molecular weight prepolymerized elastomeric or elastoplastic resins. In order to have workable viscosities and to reduce bubbling and cobwebbing during application, the resin forming liquid is usually further diluted to about 25 to 30% solids before application to the workpiece. In a 50% solids solution, solids only comprise 35% of the volume and therefore, the wet thicknes is about three times the dry thickness and multiple consecutive coatings, of at least three layers with inter-spaced drying are necessary to build up the appropriate film thickness without sagging, cobwebbing or bubbling.

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This procedure is not only time consuming but the elimination of the solvent vapors presents toxicity and flammability hazards and uniform film thickness is difficult to attain by means of the multiple coating procedure. Furthermore, bulk volume of the product is unnecessarily high and excessive solvent losses add increased expense with no compensatory advantages.

During the handling and fabrication of articles from metal sheets such as aluminum or titanium, it is imperative that the soft clad exterior surfaces that will be displayed in the final item be protected from damage by corrosion or abrasion during handling and fabrication of the article. On final inspection of an air frame assembly, it is not unusual for the inspector or the customer to require the replacement of several skins due to corrosion or abrasion blemishes. If the blemish is a surface depth only, and can be polished, a typical skin is 20 feet long by 5 feet wide and to remove just one blemish, this requires polishing the total square feet. If the blemish is too deep for polishing, it is then necessary to remove all the rivets and replace the skin.

Some temporary protection is provided 'by interleaving paper between each sheet. However, this is only effective during stack storage and the paper must be removed during handling and fabricating of the sheet. It would be most desirable to have a material that will protect at least one surface of the sheet during storage, mechanical fabrication and chemical operation such as etching, milling and laminating. Several resinous protectants are available, but are deficient in many respects. Most of the materials result in thin penetrable films that expose the surface to corrosion or abrasion. Other films are too hard and non-elastic and thus are easily fractured due to their brittleness and cannot withstand bending of the metalsheet. The most serious problems are encountered in the tendency of the films to stick to adjacent surfaces and to the metal piece itself requiring special precautions when stacking and handling and very severe and costly chemical cleaning to remove the films.

One of the most successful and widely used metal protectants is an acrylic based material. These acrylic compositions are again low solids in content and several passes are required just to build up just a few mils of these films. The acrylic films are laid down from a solvent dispersed system. The films are thermoplastic and fairly soft when hot and to protect the film, the sheets are interleaved with waxed paper during stacked storage. Moreover, the acrylic system is not wholly compatible with the metal sheets and is unpredictably temperamental as to its bond with the base sheet. Because of this, each sheet must be separably inspected in a number of areas with a strip of special tape which is applied to the film, and if the film is not sufficiently adherent to the sheet, the whole sheet is rejected and must go through the stripping and respraying line. Furthermore, horizontal stack bonding with the acrylic coated sheets is not practical since the soft acrylic film would flow or exude. When this is followed by mechanical shaping, the pattern would be impressed into the metal.

The acrylic is removed by liquid solvent such as a combination of trichloroethylene and toluene, methyl ethyl ketone or other solvent. However, the acrylic is extremely difiicult to remove and even after prolonged treatment with solvent, the surface remains hard to paint in that the paint does not sufficiently bite into the surface or fish eyes develop in the dried paint film.

When it is desired to produce a doubler, that is two or more metal sheets bonded by adhesive, the exterior surface must be protected during solvent and chemical cleaning and etching to prepare the surface for the adhesive. Furthermore, the outer surface protectant must be able to withstand the cure cycles of the adhesive. Again, the thermoplastic acrylics are not suitable because of the heatand chemicals encountered during the cure step which could cause the material to flow and/ or damage the bond of the acrylic to .the metal surface.

SUMMARY OF THE INVENTION However, according to the present invention, a resilient maskant or protectant coating having a dry thickness from to about 30 mils can be applied in a single pass without bubbling or sagging and when cured, results in a continuous film that is sufficiently adhered to the metal base so that no leakage of etchant or plating solution occurs between the film and the base, yet the film is sutliciently cohesive, that is, has sufiicieutly high tensile strength, so as to be manually peelable after the etchant or plating step is completed. The film is chemically resistant to either acid or basic etchants or salt solutions.

The film forming compositions of the invention may be sort or long pot life materials curable at room or elevated temperature. A further desired preferred characteristic of the maskants is translucency as applied but possessing the ability to turn opaqueunder mechanical stress permitting the total 'etchpattern to be either prescribed on the sub- I strate article or to be scribed onto the maskant before immersion into the etching bath.

DESCRIPTION OF THE PREFERRED EMBODIMENT It has been found that maskant or protectant films of su flicient thickness and possessing very desirable properties when cured can be applied by a single pass of a curable low molecular weight liquid elastomer or elastoplastic resin forming solution or dispersion that requires little or no volatile solvent to reduce application viscosity. The high solids low solvent solutions are possible according to the invention since the film forming compositions contain as an essential component, a liquid polymer which is modified to contain reactive end and/or side groups capable of in situ cross-linking or chain extending. to form high tensile strength continuous polymers.

The liquid prepolymers employed are preferably of the diene elastomer type, for example, polymers of a conjugated diene containing from 4 to 12 carbon atoms per molecule and preferably 4 to 8 carbon atoms per molecule, such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3- butadiene, 1,3-pentadiene (piperylene), 3-methyl-1,3-pentadiene, 1,3-heptadiene, 3-butyl-l,3-octadiene, phenyl-1,3- butadiene and the like. The conjugated dienemay also contain halogen hydroxy, carboxyl or lower alkoxy substitutents along the chain such as chloroprene, fluoroprene, 2-methoxy-1,3-butadiene, 2-ethoxy-3-ethyl-l,3-buta diene, and 2-ethoxy-3-methyl1,3-hexadiene.

The comonomer should not exceed of the polymer in order to preserve the elastomeric properties. Suitable comonomers are vinyl compounds such as'vinyl-substituted aromatic and aliphatic compounds; Examples-of comonomers that can 'be employed in the elastomer forming liquid prepolymers of the invention include acrylo-v nitrile, methacrylonitrile, propylene, butene, isobutylene, styrene, l-vinyluaphthalene, 2 vinylnaphthalene, and alkyl, cycloalkyl, aryl, alkaryl, aralkyl, alkoxy, aryloxy, and dialkylamino derivatives thereof. 1

The equivalent weight of the liquid prepolymer-is at least a thousand and not usually more than five thousand. The functionality of the prepolymer is advantageously slightly over 2, but less than 5 to form by cross-linking and chain-extending final polymers of molecular Weight of at least 20,000. With the higher molecular weight prepolymers, it may be necessary to apply heat to reduce viscosity before applying thecomposition to the substrate. Therefore, the equivalent weight is preferably from 1000 to 3000.

Functionality is provided by reactive terminal and side groups which may be at least one of thiol, (-SH) carboxyl, (COOH) hydroxyl (--OH), allylic chlorine,

(CH OHCH Cl), isocyanate N=C=O), epoxy or amine. Upon addition of polyfunctional reactive coupling agents and suitable catalysts or accelerators, the low molecular weight liquid reacts in place on the workpiece either at ambient or elevated temperature to produce a high molecular weight full'y compounded polymeric maskant. The functionality is preferably maintained within the range of 2.1 to about'-2.5 inorder that excessive cross-linking does not transform the product into too plastic a state and thus reduce the resilient properties desirable for proper chemical milling masking purposes.

The diene prepolymers preferably contain a minimum amount suitably below 40% of 1,2 addition units to avoid excessive decrease of elastomeric properties. A suitable prepolymer is a polymer of equivalent weightof about 1000-2000 having 'a functionality slightly greater than two and comprises cis 1,4 units, 20% trans 1,4 and about 10% 1,2 vinyl units.

The coupling-curing systems can include-various types of polyfunctional curatives reactive with the end or side chain functional groups. The thiol or hydroxyl modified diene liquid prepolymers can'be coupled and cured with aliphatic, aromatic or cycloaliphatic polyfunctional com pounds containing isocyanate, carboxyl anhydride, amine, acid chlorides, hydroxyl or epoxy groups.

Preferably, the polyisocyanates are those represented by the general formula R(NCO),', wherein R is a polyvalent'organic radical containing from 2 to 30 carbon atoms and m is2, 3 or 4. R can be aliphatic, cycloaliphatic or aromatic. It is preferred that the organic radical be essentially hydrocarbon in character although the presence of unreactive groups containing elements other than carbon and hydrogen is permissible.

Examples of suitable compounds of this type include benzene 1,3-diisocyanate, hexane, 1,6-diisocyanate, tolylene 2,4-diisocyanate (TDI), tolylene 2,3-diisocyanate, diphenylmethane 4,4-diisocyanate, naphthalene 1,5-diisocyanate, diphenyl 3,3-dimethyl 4,4'-diisocyanate, diphenyl 3,3-dimethoxy 4,4'-diisocyanate diethyl ether, 3(diethylamino) pentane 1,5-diisocyanate, butane 1,4- diisocyanate, cyclohex-4-ene, 1,2-diisocyanate, benzene 1,3,4-triisodyanate, naphthalene 1,3,5,7-tetraisocyanate, naphthalene, l,3,7-'t.riisocyanate,' toluidine diisocyanate, isocyanate terminated prepolymers, polyaryl polyisocyamates, and the like. I V Y A suitable commercial available polyaryl polyisocyahate is known as PAPI-l. This material has an average of 3 isocyanate groups per molecule and an average molecular weight of about 380.

Exemplary polybasicacids reactive with hydroxyl or thiol modified polymers of the invention include maleic acid, .p'yromellitic acid succinic acid, phthalic acid, terephthalic acid,- trimellitic acid, andthe like. Acyl chlorides such as phthaloyl chloride, terephthalyl chloride and fumaryl chloride, can'be utilized to couple the hydroxy groups of the prepolymer as can such compounds as .dichloromethylphosphonic dichloride, and the like.

Isocyanate modified diene polymers and those containing. allylicchlorine such as lowmolecular weight polychloroprenes are chain extended and cured with polyamines. Examples of such polyamines include tetraethylenepentamine, ethylenediamine, diethylenetriamine, triethylenetetramine, o-phenylenediamine, 1,2 propanediamine, 1,2-butanediamine, piperazine, 1,2,3-benzenetriamine, 3,3 1 biphenyl-diamine, 3,3 dichlorobenzidine, 4,4-dichlorobenzidine, 4,4-o-dichloroaniline, 4,4-methylenebischloraniline, methylenedianiline or N,Nbis(l,4- dimethylpentyl) paraphenylenediamine. The fatty diamines or amine terminatedwpolyamidessuch as can be produced by condensation of polyamines with polybasic acids can also be used.

Urethane'or esterxlinked'polymers are formed when isocyanate orcarboxylic modified" diene polymers are cured with polyhydroxy'compounds. These compounds can be either aliphatic or aromatic polyols or certain polyether products. Examples of such coupling-curing article can be modified by adding an adhesive resin to the agents include castor oil, glycerol, propylene glycol, neoformulation in an amount to give the desired adhesion. pentylglycol, pentaerythritol, trimethanolethane, trimeth- The amount utilized will vary with the amount of film anolpropane, butanediol or hexanetriol. forming diene prepolymer present and the type and con- It is thus seen that an essentially diene elastomer is dition of the surface being treated. However, excessive formed of a plurality of prepolymer elastomeric polyamounts should be avoided since that may cause improper diene units joined by coupling reagents which condense hand-stripping properties. Use of amounts in the ranges to form linking urethane, thiourethane, ester, urea alkyl given below results in satisfactory adhesion with good urea, thiourea, aminoalkyl units or combinations thereof. stripping qualities. The preferred adhesive resins are Generally, the coupler is present in the range of 50 to phenolics, epoxies or other resins of the thermosetting 150% of stoichiometric based on the functionality of the type, suitably an alkyl phenolaldehyde resin such as a prepolymer. Polymers can be cured at temperatures from butyl or t-butyl phenolaldehyde resin. Many commercial ambient to 500 F. although preferably they are curable forms of these resins are available such as Durez 13355 in the range of 50 F. to 100 F. The time of cure can (Hooker Electro Chemical) or Epon 1001 (Shell). be anywhere from several minutes to several days, again 5 The pot life of the formulation, speed of curing and depending upon the polymer being cured, the coupler and properties of the final polymer are also influenced by the the temperature of the curing reaction. Curing can be choice of accelerator and activator. The composition may accelerated by appropriate agents. further contain small amounts of conventional diene The curing is carried out after the prepolymer has been curatives such as sulfur compounds or free radical gencompounded with pigments, extenders, accelerators and crating agents such as a peroxide or an azo compound an optional resinous adhesive material and worked up such as diazobicyclooctane. The accelerator may further with solvent to a smooth liquid state. For short pot life comprise an alkyl amine such as triethylamine, heavy materials, the coupling agent and the accelerator are not metal esters such as tin dibutyldilaurate or stannous added until substantially immediately before the liquid octoate. A metal oxide such as zinc or magnesium oxide is applied to the metal part. may be included as an activator.

In addition to the diene liquid prepolymer and the cou- The following are typical urethane or urethane-urea pling agent, the film forming compositions typically can linked diene elastorner formulations suitable for the in also contain an antioxidant such as a hindered phenol, or situ single application method of this invention: an aromatic amine such as phenylalphanaphthylamine, a

TABLE I moisture scavenger such as activated alumina, silica or sodium alumina silicate, and an aromatic or polar plas Example (ph ticizer. Exemplary plasticizers are hydrocarbon oils such 1 2 as a naphthenic oil, a chlorinated hydrocarbon oil such as Polymer an Arachlor product or a polyether or an ester such as Liquid polybutadicne on 100 dibutyl or dioctyl phthalate or tricresyl phosphate. A gg Styrene 00 quantity of aromatic or low molecular weight solvent Li idifiigiiri iiibflflbifi A Such as 0r methyl :P ketone (ME?) may be nilllfdgfisnt'asstzsrniijjjjjjjjj:::::::::: it"? "i516 present if required. All additives are present in amounts Filler 10-100 5-100 5-100 5-100 within ranges specified below to enhance the chemical -50 0-50 Accelerator 0. 01-. 06 0. 01-0. 6 and physical properties of the maskant. 40 i v etai oxide activaton- 0-10 0-10 0-10 0-10 oisture scavenger 0-10 0-10 0-10 0-10 Carbon black, titanium dioxide 811168. or various mm Adhesiveresm 5&5 1H5 F25 eral silicate or carbonate fillers can be added to improve lgritioxidant 0-2 oz 0-2 0-2 o vent 5- 50 5-50 5-50 5-50 the tensile strength of the resultant film. However, with C0upling cmmg agent (polyiso opaque filled maskants, it is difiicult to see scribe lines, cyanate).. 10-25 10-25 10-25 10-25 surface imperfections, abrasions or bubbles. It is there- Org Org fore preferred that the final maskant be translucent as applied, but capable of being rendered visible in regions e above polybutadlenes have an equivalent weight of mechanical stress. These objectives can be secured by of from about 1000 to 2000 and a functlonallty 0f Shghtchoosing fillers whereby the difference in refractive index ly over 2. When cured at 77 F. and RH, the polyof the filler n (when contacted by the matrix polymer 50 mers have the following properties:

TABLE II Example Pot-life (min) 2-30-. 2-30 2-30. Tack free time (min.) 15-60 15-4 -90 Cure time to develop rubbery film (m 60. 120. Tensile (p.s.i.) 7001,000-- 500-800. Elongation (percent) 200-300 200-400 150-200 200-300. Chemical resistance:

6 hrs. in 10% caustic at 190 F No efiect No effect N0 efiect 30% loss in tensile.

6 hrs. in 10% B01 at 130 F 30% loss in tensile 207 30 0 Pol ymcr disintegrated.

6 hrs. in 10% HNO; at 130 F 30?, loss in tensile.

' 6 hrs. in boiling Water Peel adhesion-clad Al (lbs) 0.75-2.

of the maskant) and the matrix n is less than 0.5 in ac- It has further been found that the addition of certain cordance with the teaching of my earlier Patent No. amounts of diamines to polyisocyanate coupled hydroxyl 3,227,589. Preferred translucent pigments are calcium, or thiol modified polymer systems results in urethane-urea magnesium, barium or aluminum silicates, although SiO or thiourea linked final polymers of improved chemical or calcium, magnesium or zinc carbonates or others may resistance. For example, if the formulation of Example 1 be incorporated into the film. These fillers are usually is modified to contain 0-9 phr. and preferably less than present in the range of 10 to 100 phr. and preferably 25 5 phr. of diamine corresponding to a NH zOH ratio of to 50 phr. less than 1 and preferably less than 0.5, the following Adhesion of the maskant or protectant to the metal changes of properties result, when cured with TDI at 77 7 F., 510% RH. for 24 hours with a NCOzNH -l-oH ratio of 1.1.

TABLE 111 Example (1 shot method) 1 1a 1b 1c 1d 1e 1i Diamine (phr.):

3,3-dich1orobenzidine 1 2 3 4 5 6 4,4-methylenebischloroani 'ne N,N-bis (1,4 dimethylgantyl) paraphenylcne- Elongation (percent). 200 200 250 300 250 200 175 300% modulus 750 Percent tensile change 10% caustic at 190 F. for 6 hrs 10 0 0 0 30 Certain diisocyanates are found to give better overall physical, chemical and adhesive properties in the resultant film. In a series of experiments with hydroxy modified polybutadiene having a 0.1 excess of isocyanate cured at 77 -F. and 50% for 24 hours the following results were recorded:

TABLE IV Example Properties:

Tensile, p.s.i Elongation, percent 300 350 250 300 350 Percent tensile change in 10% caustic, 100 F. for 6 hrs 20 It is apparent that films with polymeric polyaryl isocyanates and toluidine isocyanates form films with optimum properties. The above isocyanate compositions are room temperature curing. Heating to 250 F. results in a 50% drop in elongation, 20-50% increase inmodulus, virtually no change in tensile and a considerable improvement in chemical resistance.

The pot life of these compositions can be substantially lengthened by the use of chemically blocked polyisocyanates that are activated by heat or moisture. Suitable blocked polyisocyanates are the phenol blocked isocyanate materials such as Mondur SH, ('Mobay) or Hylene MP (Dupont) phenol blocked MDI or Isonate 123P (Upjohn). Another way of permitting premixing of the ingredients of the film including the curing agent is to physically block the reactive ingredients. For example, the curing agent can be enclosed in the micro voids of a molecular sieve or enclosed in the shells of a microencapsulating material. The curing agent is activated by release from its physically blocked state by means of heat or moisture.

The maskant or protectant film forming materials of the invention can be applied by spraying, forced flowing, dipping or brushing a coating of the composition onto a substrate such as an aluminum, magnesium, ferrous, beryllium or other metal alloy and the film is cured during and after application to the substrate. Suitable equipment that can be utilized is a two component dispersing type spray gun in which the compounded polybutadiene is meteredat a desired ratio and the curing agent is simultaneously metered and mixed in a small chamber seconds before being sprayed. The mixed components are deposited as a smooth uniform film that cures within 1-24 hours at ambient temperature or with heat if required.

The deposition of a smooth high solids film that is cured in place may also be effected with a modified prepolymer method in which the short chain liquid prepolymer is pre- 8 reacted with an excess of coupling agent to form a polymer product with- 2-9% of available functional groups from the coupling agent. The prepolymer is then compounded with pigments, adhesive resin, solvent and accelerator similar to the one-step procedure. Final curing is effected by adding a further coupling agent.

For example, a hydroxyl or thiol modified polybutadiene may be reacted with an excess of toluene diisocyanate to form 29% of 'NCO groups. The prepolymer is compounded and then cured with a diann'ne, a diol, a triol or even some additional hydroxyl or thiol modified polybutadiene or mixtures thereof may be utilized as chain extending and/or cross-linking agents.

Again, the properties are dependent somewhat on the choice and concentration of coupling and accelerating agents and the properties are again substantially improved with the preferred NH :OH ratio of less than 1.

A series of compositions were prepared to show that within the stated ranges, a wide variety of accelerators will provide suitable maskant or protectant films.

EXAMPLE 1 To one hundred parts of a hydroxyl modified polybutadiene having an equivalent weight of 1300 and a functionality of slightly over two, were added 22 phr. of TDI and after reaction to form a polymer product containing 6% NCO, the composition was compounded with filler, plasticizer oil, solvent and moisture scavenger and antioxidant in amounts previously specified. The composition was then cured for 24 hours at 77 F., 50% 'R.H. with the following catalysts. The curing-coupling agent in each case was 1,2-butane-diol and 3% of 4,4-dichlorobenzidine.

Due to the resistance of the maskant or protectant film of the invention to both acidic and basic chemicals and further because of the adherence to the substrate to avoid leakage. even after adjacent areas have been cut and peeled, the films also are very effective as maskants. in processes for the selective addition of metal whether from electroless or electrolytic baths. The maskants, for example, can be utilized in the plating of nickel, copper, gold, silver, cobalt or zinc onto metal bases such as alloys of aluminum, magnesium, ferrous, beryllium or other metals.

The following procedures are illustrative of techniques to be followed in the utilization of the maskants of the invention in selective plating onto metal substrates.

EXAMPLE 2 Chromate conversion coating G./l. 'Chromic anhydride 5 Potassium ferricyanide 1 Barium nitrate 1.9

Sodium 'fluosilicate 1.35

A chromium conversion coating was deposited on the panel in the exposed areas. The panel was removed from the plating bath, drained, rinsed and dried at ambient conditions. The remainder of the maskant film was removed from the panel by peeling and the plated pattern was clearly defined and the maskant covered areas showed no sign of plating or attack by the plating solution.

This procedure .was repeated with a steel panel coated with maskant and then selectively stripped in areas before being immersed in an electrolytic chromium bath containing 250 g./l. chromic acid with a sulfuric acid concentration of 1 to 1.5% at a current density range'of about 500 amp/ft. and a temperature of 122-131 F. A bright, hard, clearly defined chromium deposit was produced on the maskant stripped areas, the remainder of the plate being free of corrosion or deposition.

The properties of the film forming composition of the invention also provide many time, material and procedure having advantages when utilized as a peelable metatl protectant. Furthermore, new fabrication procedures become possible because of its multiple properties. The single pass application and short cure time alone saves 75 to 80% compared to the acrylic coating procedures. The film of the invention contains little solvent and does not present a sol-vent sticking or otherwise adhesive exterior and thus may be readily handled and stacked without difficulty.

The film of the invention is not attached to the metal, but is only peelably adhered thereto and can be simply stripped by hand or can be removed in a simple automated procedure by subjecting the film coated sheet to solvent vapor and the film will fall away from the sheet. Even though the film of the invention is only peelably adhered to the substrate, all exposed surfaces of the film exhibit extreme resistance to attack by acidic or caustic chemicals. Furthermore, the metal-film bond is not disturbed by the mechanical pressure encountered in rolling, bending or shaping the sheet. Nor is the material affected by fairly high temperature treatment of the order of 200 to 400 F. whether applied by radiation or direct steam treatment. These qualities combined to make the film of the invention a totally versatile metal protectant which provides continuous and multiple types of protection during storage, handling, mechanical shaping and etching and adhesion of metaal sheets to form laminates as will be described. Furthermore, by selective removal of the film and treatment of the exposed surface, lap bonding of the material becomes possible as does chemical milling of the laminates to provide desired structural shapes or Weight reduction of the composite element.

During use of the film forming composition of the invention as a metal protectant, the composition is applied to the surface preferably in a single pass and is cured thereon to form a durable, peelably adhered protectant film. The coated sheet may then be subjected to mechanical shaping and is then fastened to other sheets by riveting, welding, or by lap bonding as will be described. The film can be removed before bonding or may be maintained on the sheet until after the rivets are secured and may be simply stripped away by handor subjected to a vapor of solvent to loosen attachment of the film to the sheet before it is stripped away.

When it is desired to form laminates, at least one sheet of a set of metal sheets is subjected to a single spray pass of the composition of the invention which is cured to form a film. The uncoated side of the sheets is then subjected to preliminary cleaning such as vapor degreasing and then to chemical cleaning and etching. The adhesive is then applied to the etched surface which may be precoated with an adhesive primer. The two sheets are assembled into a composite and the adhesive cured, typically by placing the doubler in an autoclave. The sheets may be mechanically shaped before or after bonding. The bonded laminate is then attached by secondary fastening to another sheet by suitable means as discussed above and the metal protectant film is finally removed to expose a clear, unmarked and unblemished surface.

When it is desired to prepare such sheets for lap bonding after removal of the laminated sheets from the curing oven, a portion of the protectant film is scribed with a sharp knife and stripped from the metal sheet. This procedure is followed with at least two sets of laminated sheets. The sheets are returned to the metal cleaning and etching stations and are cleaned and etched to render the surfaces receptive to the adhesive. Adhesive is applied to each etched surface and the surfaces are joined and placed in the autoclave. After removal from the autoclave, the assembled pieces can be joined to other structures by secondary bonding techniques and the film removed. An example follows:

EXAMPLE 3 An aluminum (2024-T3) panel (10 cm. x 15 cm.) was prepared by degreasing, caustic etching for 30 seconds at 70 C., rinsing, treating the panel with 50% by volume HNO and then rinsing again.

After the panel was dried, a 10 mil film of the hydroxy polybutadienepolyisocyanate and diamine cured maskant film of Example 3 of Table IV was applied to the panel by spraying. The procedure was repeated with an identical pane Both panels were prepared for application of adhesive by'vapor degreasing, caustic etching, rinsing and a deoxidizing bath consisting of a sulfuric acid-sodium dichromate bath and were then dried. Each panel was then coated with an epoxy adhesive primer and then with an epoxy adhesive, in this case one manufactured by the Bloomingdale Corporation. The panels were placed in a vacuum to remove bubbles from the adhesion and were then cured in a steam autoclave for one half hour at p.s.i., 250 F. The laminate was removed from the autoclave and cooled to room temperature.

A line was scribed into the protectant film with a sharp knife two inches from the end of two laminates, and the film removed to expose the substrate metal. The two laminates were returned to the metal cleaning station and subjected to solvent cleaning, caustic cleaning and sulfuric acid-sodium dichromate deoxidizing treatment. The deoxidized surfaces were coated with primer and adhesive and then joined and returned to the autoclave for a curing cycle. The joined doubler composites were removed and cooled, and were found to be firmly bonded. After two cycles of metal cleaning and curing, the film of the invention was still found to be peelably and firmly adhered to the metal.

Patterns were scribed with a sharp knife in the bottom surface of the lapped composite member and the film was removed from these areas. The lap composite was then placed in an aluminum etching bath and the metal was etched from the exposed areas to the adhesive layer to lighten the member. The lap bonded member can then be assembled by riveting as part of an airplane fuselage by riveting and the film removed by peeling or before assembly, the film can be removed by a few minutes of contact with the vapors from a vapor degreaser and the film will either fall away or can be removed with a very light peeling force.

At every step of the processing, considerable economies and conveniences are realized when utilizing the film forming composition of the invention. The film of the invention is applied in a single pass of a spray gun and can be cured at moderate heat at about 50 to 200 F. in a few minutes to several hours. However, the acrylic metal protectants must be applied several times, and usually about 7, to build up a coat of equal thickness so that of the invention and curing at F. to 200 F. requires very long periods. The acrylics are sticky and must be handled with care while the elastomeric or elastoplastic films of the invention are non-sticky and can take considerable rough handling without breaking, tearing or allowing the substrate surface to be marred.

Furthermore, the film of the invention is consistently and regularly applied successful to an even film thickness of constant adhesion and peelable quality. However, the acrylics are unpredictably applied in terms of adhesive power which may be more or less than that desired. The films of the invention strip easily and readily to provide a surface that accepts paints readily while with the acrylics, severe solvent removal treatment is required and even after removal, the surface still does not provide a good bite for paint. At one air frame manufacturing facility alone, the cost of maintenance of the liquid solvent tank for removal of the acrylic film is one-half of a million dollars per year.

From the foregoing, it is evident that the compositions of the invention can be modified substantially to tailor made maskants or protectants of desired ambient or high temperature curing properties and varying physical properties suitable for particular chemical milling, laminating, plating or metal protectant applications. The high solids, low solvent compositions of the invention are efficiently applied in a single step to form a uniform film of to 30 mil thickness without bubbling or cobwebbing and avoiding the delays, expense and hazard of multiple application and evaporation of excessive amounts of solvent. The final films are adherent, abrasion resistant, extensible, resistant to etchant or plating bath chemicals and are cuttable and peelable to expose the surface to be etched, plated or otherwise processed. Hence, it is apparent that the invention constitutes a marked improvement over prior art compositions and techniques.

It is further to be understood that preferred embodiments have been disclosed and that the numerous modifications and alterations are possible without departing from the scope of the invention defined in the claims that follow.

What is claimed is:

1. A composition for applying in a single application to a substrate an adherent, high molecular weight, continuous, manually peelable, etchant resistant film up to 30 mils in thickness having a tensile strength of from 400 to 1000 p.s.i., an elongation of from 150 to 450% and a pull adhesion to aluminum of from 0.75 to 3 pounds, said composition containing:

a low molecular weight, liquid diene prepolymer resin having an equivalent weight of from 1000 to 5000 and containing between 2 to 5 end and side chain functional groups on each prepolymer molecule selected from the class consisting of hydroxyl and thio modified diene homopolymers and diene copolymers;

-25 parts by weight per one hundred parts of said resin of a coupling-curing agent condensible with the functional groups on said polymer and being at least one member selected from the group consisting of a polyisocyanate, and a polyamine to form a film having said properties;

10-100 parts by weight per one hundred parts of said resin of a filler having a refractive index difiering by less than 0.5 from the refractive index of said film and being selected from the group consisting of a magnesium, calcium or aluminum silicate, carbonate or oxide;

5-80 parts by weight per one hundred parts of said resin of volatile solvent; and

525 parts by weight per one hundred parts of said resm of a thermosetting adhesive resin selected from the group consisting of phenolic and epoxy thermosettmg adhesive resins.

2. composition according to claim 1 in which the functionality of the propolymers is from about 2 to 3.

3. A composition according to claim 1 containing hydroxyl or thiol modified diene liquid prepolymers of equivalent weight of from about 1000 to 2000 and a functionallty of greater than 2 and less than 3 and the coupling agent 1s a combination of a polyisocyanate and a polyamine such that the ratio of NH :OH or SH is less than 1 indAthe NCO: l IH +OHdor SH ratio is from 0.8 to 1.2.

composi 101'1 accor in to claim NH :OH ratio is less than 0.5. 3 m whlch the 5. Apomposition according to claim 3 in which said polyamine is a diamine selected from the group consisting of 3,3 -d1chlorobenzidine, 4,4'-methylenebischloroaniline, N,Nb1s( 1,4 dimethylpentyl)paraphenylenediamine and methylene dianiline.

6. A composition according to claim 1 in which the diene copolymers contain notmore than 35% of a vinyl co-monomer.

7. An article prising:

a base metallic article; and

a chemical milling maskant peelably adhered thereto, said maskant being the cured reaction product of a liquid diene prepolymer and a polyfunctional couplmg curing agent as defined in claim 1.

8. An article of manufacture comprising:

a mectlal object having at least one exterior finish surface;

an elastomeric resinous film of from 5 to 30 mil thickness peelably adhered to said surface, said resin being the reaction product of a polyfunctional liquid diene prepolymer having a functionality from 2-5 and a polyfunctional coupling-curing agent condensible therewith as defined in claim 1. '7

9. A composition according to claim 1 in which the polyisocyanate coupling-curing agent is selected from polymerlc polyaryl isocyanates and toluidine isocyanates.

10. A composition according'to claim 1 in which the polyisocyanate coupling-curing agent is a phenol blocked isocyanate. i v I References Cited UNITED STATES PATENTS tobe chemically milled or plated com- 3,'084,14l 4/ 1963 Kraus et al 2'6094.7 N 3,227,589 l/l966 Deutsch 1l7-6 3,317,479 5/1967 'Noshay et al. 260-94.7 OX

H. H. FLETCHER, Assistant Examiner US. Cl. X.R.

1l7--132 CB; l5613; 260--77.5 AP, 78 CA, 94.7 R, 94.7 A, 94.7 N. 

